Method and apparatus for producing metal by electrolysis of molton salt

ABSTRACT

A process for production of a metal includes a step of filling a metal chloride in an electrolysis vessel having positive and negative electrodes, a step of heating and fusing the metal chloride to make an electrolytic bath, and a step of electrolyzing the electrolytic bath to deposit metal on the negative electrode in a solid state. In addition, in an apparatus for production of a metal in which a metal chloride is filled in an electrolysis vessel having positive and negative electrodes, the metal chloride is heated and molten to make an electrolytic bath and the electrolytic bath is electrolyzed to deposit the metal on the negative electrode in a solid state, the electrolytic bath is divided into an electrolysis chamber and a dissolution chamber by a dividing wall, the positive electrode is arranged in the electrolysis chamber, the negative electrode is arranged to enable orbital movement in a circle through the electrolysis chamber and dissolution chamber, and the metal deposited on the negative electrode in the electrolysis chamber is separated and recovered in the dissolution chamber.

TECHNICAL FIELD

The present invention relates to the recovery of metal from a chloride thereof, and in particular, relates to a method and an apparatus for producing metal by electrolysis of molten salts containing metal chlorides.

BACKGROUND ART

Conventionally, titanium metal, which is a simple substance, is produced by the Kroll method in which titanium tetrachloride is reduced by molten magnesium to obtain sponge titanium, and various kinds of improvements have been made to reduce the cost of production. However, since the Kroll method is a batch process in which a set of operations is repeated noncontinuously, there is a limitation to its efficiency.

To resolve such a situation, a method in which titanium oxide is reduced by calcium metal in molten salt to obtain titanium metal directly (see WO99/064638 and Japanese Unexamined Patent Application Publication No. 2003-129268), one in which an EMR method in which a reducing agent containing an active metal such as calcium or an active metal alloy is prepared, and one in which a titanium compound is reduced by electrons emitted from the reducing agent to yield titanium metal (see Japanese Unexamined Patent Application Publication No. 2003-306725) have been proposed. In these methods, calcium oxide, which is a by-product of the electrolytic reaction, is dissolved in calcium chloride, and molten-salt electrolysis is performed to recover and reuse calcium metal. However, since the calcium metal generated during the electrolytic reaction is in a liquid state and has high solubility in calcium chloride, it dissolves easily in the calcium chloride. There has been no disclosure of a technique to recover calcium metal in a solid state alone.

Furthermore, a technique in which a molten salt electrolysis is performed at a temperature lower than the conventional electrolysis using a complex molten salt having a melting point lower than that of calcium metal to deposit calcium metal on a negative electrode in a solid state is disclosed (see U.S. Pat. No. 3,226,311). However, in this production method, it is necessary to prepare the complex molten salt specially, and the cost is considerable.

As explained above, there is a problem in that it is difficult to recover an active metal such as calcium metal alone, and there is a problem in that the cost is high even if the recovery is possible.

The present invention has been completed in view of the above situation, and an object of the present invention is to provide a method for production of metal by molten-salt electrolysis, in which a metal used for reducing, such as an oxide or chloride of titanium metal, is recovered in a solid state and at low cost.

DISCLOSURE OF THE INVENTION

That is, a method for production of metal by molten-salt electrolysis of the present invention has a step of filling an electrolysis vessel having a positive electrode and negative electrode with a metal chloride, a step of heating to fuse the metal chloride to make an electrolytic bath, and a step of electrolyzing the electrolytic bath to deposit metal in a solid state on the negative electrode.

By the present invention, a metal can be deposited on the negative electrode in a solid state, which is a state having low solubility in the molten salt, and it can be recovered. Furthermore, the recovery of the metal can be performed at low cost.

An apparatus for production of metal by molten-salt electrolysis of the present invention has an electrolysis vessel having a positive electrode and a negative electrode therein and a metal chloride filled in the vessel, the metal chloride is heated and molten to make an electrolytic bath, and the electrolytic bath is electrolyzed to deposit metal in a solid state on the negative electrode. Furthermore, in the apparatus, the electrolytic bath is divided into an electrolysis chamber and a dissolution chamber by a dividing wall, the positive electrode is arranged in the electrolysis chamber, and the negative electrode is arranged to enable orbital movement in a circle through the electrolysis chamber and the dissolution chamber. Metal which is deposited on the negative electrode in the electrolysis chamber is recovered in the dissolution chamber.

By the present invention, electrolysis of metal chloride is promoted and the metal is deposited on the negative electrode while the negative electrode is passing through the electrolysis chamber, and the metal deposited can be recovered during the negative electrode passing through the dissolution chamber. Furthermore, since the negative electrode revolves and passes through the electrolysis chamber and dissolution chamber regularly, deposition and recovery of the metal can be automatically and efficiently performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram showing the method for production of calcium metal in an embodiment of the present invention.

FIG. 2A is a conceptual diagram showing the method for production of calcium metal in another embodiment of the present invention, and FIG. 2B is a conceptual diagram showing a scraping device on the negative electrode in the case in which FIG. 2A is seen from direction A.

FIG. 3A is a conceptual diagram showing the method for production of calcium metal in another embodiment of the present invention, and FIG. 3B is a conceptual diagram in the case in which FIG. 3A is seen from direction A, and FIG. 3C is a conceptual diagram in the case in which FIG. 3A is seen from direction B.

EXPLANATION OF REFERENCE NUMERALS

-   1 . . . Electrolysis vessel, -   1 a . . . Electrolysis chamber, -   1 b . . . Dissolution chamber, -   2 . . . Electrolytic bath, -   3 . . . Positive electrode, -   4 . . . Negative electrode, -   5 . . . Metal (calcium), -   6 . . . Molten salt, -   7 . . . Recovery vessel, -   8 . . . Chlorine gas, -   9 . . . Scraping device, -   10 . . . Dividing wall

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are explained below with reference to the drawings. FIGS. 1 to 3 show embodiments of an apparatus to perform the present invention. Next, a case in which the electrolytic bath is one of calcium chloride and the metal generated is calcium metal, is explained.

In FIG. 1, reference numeral 1 is an electrolysis vessel, and electrolytic bath 2 comprising calcium chloride is filled therein and heated to a temperature not less than the melting point of calcium chloride by a heating means, which is not shown, to keep the electrolytic bath in a molten state. Reference numeral 3 is a positive electrode and reference numeral 4 is a negative electrode, and they are immersed in the electrolytic bath 2.

The positive and negative electrodes 3 and 4 are connected to a direct current power supply, which is not shown, and electrolysis of the electrolytic bath is started. Chloride ions in the electrolytic bath 2 are attracted to the positive electrode 3 and emit electrons to generate chlorine gas which is lost from the system. Calcium ions are attracted to the negative electrode and receive electrons to generate calcium metal 5 which is deposited on the surface of negative electrode 4.

The present invention can be efficiently performed even in the cases in which the temperature of the electrolytic bath 2 is above or below the melting point of calcium metal 5 (845° C.). In particular, if the temperature of the electrolytic bath 2 is below the melting point of calcium metal, calcium metal 5 can be deposited in a solid state on the surface of the negative electrode 4. On the other hand, even if the temperature of the electrolytic bath 2 is above the melting point of calcium metal 5, calcium metal 5 can be deposited in a solid state on the surface of the negative electrode 4 in the case in which a cooling structure is installed in the negative electrode 4.

In both cases, since calcium metal 5 generated in the electrolytic reaction is deposited in a solid state, the solubility of the solid calcium metal 5 in calcium chloride contacting therewith is extremely small. Therefore, calcium metal can be recovered at high yield.

After a certain amount of calcium metal 5 is deposited on the negative electrode 4, the negative electrode 4 is taken out of the electrolytic bath 2 and is immersed in a recovery vessel 7 having molten salt 6 for which the temperature is maintained above the melting point of calcium metal 5 (845° C.). The calcium metal 5 deposited on the negative electrode 4 is partially dissolved in the molten salt 6 held in the recovery vessel 7, and the rest floats up from the negative electrode 4 to be condensed around the surface of the liquid. The condensed part is collected and recovered. In this case, since evaporation loss becomes larger as the temperature is increased, the temperature is practically set at not more than 900° C.

Calcium can be dissolved, floated and recovered in a liquid state as explained above, and in addition, it can be cooled to not more than the melting point of calcium metal (845° C.) as long as the molten salt 6 is not solidified. By performing such a cooling operation, calcium metal 5 floats in a solid state, and it can be efficiently recovered. Since the melting point of calcium chloride is about 780° C. and the melting point of calcium metal is about 845° C., by decreasing the temperature of the recovery vessel 7 to about 800° C., calcium metal 5 molten in the molten salt 6 can be recovered in a solid state.

The negative electrode 4, after separating and recovering the calcium metal 5 deposited, can be transferred from the recovery vessel 7 to the electrolysis vessel 1 to perform molten-salt electrolysis again. By repeating the above-mentioned set of operations, calcium metal can be efficiently recovered. The calcium metal 5 generated in the recovery vessel 7 in this way can be used in the reduction of titanium tetrachloride using molten salt.

Alternatively, calcium metal is not recovered in the recovery vessel 7 leaving the concentration of calcium metal in the molten salt 6 high, and the molten salt containing calcium can be used in the reduction reaction of titanium tetrachloride.

Chlorine gas 8 generated on the positive electrode 3 in the electrolysis vessel 1 can be separately recovered and can be reused in a chlorination reaction of titanium ore. Alternatively, it can be used for other purposes.

As a material of the positive electrode 3, a material having an electrical conductivity which does not dissolve in the electrolytic bath and does not react with chlorine gas, is desirable. As such a material, carbon is desirable.

The negative electrode 4 can be constructed by an electrical conductive material, for example, carbon steel, stainless steel, copper, aluminum or the like can be used. The negative electrode 4 desirably has a structure in which a cooling medium can be circulated therein. The structure can promote deposition of calcium metal on the negative electrode 4.

As the molten salt 6 in the recovery vessel 7, arbitrarily selected one can be used, and in particular, calcium chloride is desirable. Since calcium chloride is a by-product of molten-salt electrolysis of titanium chloride and calcium metal, if the molten salt 6 is calcium chloride, it will be unnecessary to remove calcium chloride when condensed calcium metal is used in a molten-salt electrolysis process for titanium chloride. In addition, that is because after the molten-salt electrolysis process for titanium chloride, the molten salt 6 can be reused with calcium chloride which is a by-product of this process in the electrolysis vessel 1.

The melting point of the electrolytic bath can be decreased by adding potassium chloride to calcium chloride forming the electrolytic bath 2. The amount of potassium chloride added to calcium chloride is desirably in a range from 20 to 80 wt %. By adding potassium chloride in this range, even if the temperature of the electrolytic bath 2 is decreased below the melting point of calcium metal by the temperature between 150° C. and 250° C., reliable operation can be performed without solidification of the electrolytic bath.

The temperature of the electrolytic bath 2 can be arbitrarily controlled within the target temperature range by using a heating burner having a cooling function, which is not shown, immersed in the electrolytic bath. Alternatively, another means can be employed to control the temperature of the electrolytic bath 2.

FIG. 2 shows another embodiment of the present invention. Electrolytic bath 2 comprising calcium chloride is filled in the electrolysis vessel 1 of FIG. 2A, is heated to a temperature not less than the melting point of calcium chloride by a heating means, which is not shown, and is held in a molten state. Furthermore, the positive electrode 3 and the negative electrode 4 having cylindrical shape are immersed in the electrolytic bath 2. This negative electrode 4 can be constructed so as to be rotatable, and the scraping device 9 is arranged neighboring to an edge of a side surface of the cylindrical negative electrode 4. FIG. 2B shows a conceptual diagram of the negative electrode 4 and the scraping device 9 seen from the direction A. As shown in the figure, by rotating the negative electrode, calcium metal 5 deposited on the surface of the negative electrode is efficiently scraped by the scraping device 9.

Solid calcium metal 5 scraped from the negative electrode 4 floats up to the surface of the electrolytic bath 2 since the density of calcium metal is lower than that of calcium chloride. The calcium metal 5 which floated to the surface of the electrolytic bath 2 is recovered from the electrolytic bath 2. The solid calcium metal recovered from the electrolytic bath 2 is used as a reducing agent for titanium oxide in molten-salt electrolysis.

In this case, a basket having a net structure can be arranged around the scraping device 9. By taking the basket out of the electrolytic bath at appropriate times, solid metal deposited can be efficiently recovered.

It is desirable that the dividing wall 10 be arranged around the surface of electrolytic bath 2. Calcium metal deposited on the negative electrode 4 is scraped and then floats and diffuses to the bath surface. Finally, calcium metal can reach the positive electrode 3, and it has a tendency to react oppositely with the chlorine gas generated on the positive electrode 3. However, by arranging the dividing wall 10, diffusion of floating calcium metal can be prevented, and the back reaction can be effectively suppressed.

The temperature of the electrolytic bath around the scraping device 9 can be maintained, to a limited extent, at a temperature not less than the melting point of calcium metal, by immersing and arranging a heater near the scraping device 9. In this way, calcium metal scraped from the negative electrode 4 can be recovered in a molten state. The calcium metal in a molten state is partially dissolved in calcium chloride, and the rest floats up in the electrolytic bath 2. Therefore, calcium chloride having condensed calcium metal is floating around the surface of the electrolytic bath 2 via the dividing wall 10. By extracting floating calcium chloride having condensed calcium metal, for example, it can be used in reduction reactions for titanium tetrachloride.

FIGS. 3A to 3C show another embodiment of the present invention. FIG. 3B is a conceptual diagram of FIG. 3A seen from direction A, and FIG. 3C is a conceptual diagram of FIG. 3A seen from direction B. Electrolytic bath 2 comprising calcium chloride is filled in the electrolysis vessel 1 of FIG. 3, and the electrolytic bath 2 is heated to a temperature not less than the melting point of calcium chloride so as to be maintained in a molten state by a heating means, which is not shown. Furthermore, the positive electrode 3 and the negative electrode 4 are immersed and arranged in the electrolytic bath 2. The electrolysis vessel 1 is divided into the electrolysis chamber 1 a in which the positive electrode 3 is immersed and the dissolution chamber 1 b is isolated by the dividing wall 10 arranged around the surface of the electrolytic bath 2. It should be noted that only the upper part of the electrolytic bath 2 is divided by the dividing wall 10, and the lower part thereof is unified. As shown in FIG. 3C, plural negative electrodes 4 are arranged to enable orbital movement in a circle through the electrolysis chamber 1 a and dissolution chamber 1 b. These negative electrodes 4 can be revolved in a circle through the electrolysis chamber and dissolution chamber by passing through a sliced channel arranged at a part of the dividing wall 10.

Heating function and cooling function are provided to the negative electrode 4. That is, a flow passage in which a heater and a cooling medium can be circulated is arranged inside the negative electrode 4. In this way, the temperature of the negative electrode 4 can be arbitrarily controlled from a temperature not more than the melting point of calcium metal 5 to a temperature not less than the melting point of calcium metal 5.

In the construction of apparatus explained above, when the negative electrode 4 is in the electrolysis chamber side, the temperature of the negative electrode 4 is maintained at a temperature not more than the melting point of calcium metal to deposit the calcium metal on the surface of the negative electrode in a solid state. On the other hand, when the negative electrode revolves and reaches to the dissolution chamber side, the temperature of the negative electrode 4 is maintained at a temperature not less than the melting point of calcium metal to fuse the calcium metal deposited. Calcium metal 5 is molten and released from the negative electrode 4 and is partially dissolved in calcium chloride, and the rest floats up in the electrolytic bath, to form a calcium metal condensed layer. The calcium metal condensed layer formed at the bath surface of the dissolution chamber of the electrolytic bath 2 is extracted at appropriate times, and for example, it can be used as a reducing agent for titanium oxide in molten-salt electrolysis.

In this way, by controlling the temperature of the negative electrode from a temperature below the melting point of calcium metal to a temperature above the melting point of calcium metal depending on the position of the negative electrode 4 revolving inside the electrolytic bath 2, calcium metal can be efficiently recovered.

EXAMPLES Example 1

Molten-salt electrolysis of calcium chloride was performed by using both the electrolysis vessel and the recovery vessel shown in FIG. 1. Calcium metal is deposited on the negative electrode by controlling the temperature of the electrolytic bath comprising calcium chloride at 800±5° C. As a result, calcium metal having 85% of the weight of the theoretical weight calculated from electricity applied between the positive electrode and negative electrode was recovered.

Example 2

Using the apparatus shown in FIG. 2 and using calcium chloride as an electrolytic bath, molten-salt electrolysis was performed to deposit calcium metal continuously on the negative electrode. The calcium metal was scraped by the scraping device to recover it in a solid state. The amount of calcium metal produced per unit time was about twice as much as that of Example 1.

Example 3

Using the apparatus shown in FIG. 3 and using calcium chloride as an electrolytic bath, molten-salt electrolysis was performed to deposit calcium metal continuously on the negative electrode. The calcium metal in a molten state partially containing the electrolytic bath was recovered. The amount of calcium metal produced per unit time was about twice as much as that of Example 2.

As explained thus far, calcium metal can be efficiently produced by electrolysis of calcium chloride by the present invention. 

1-10. (canceled)
 11. A process for production of a metal by molten-salt electrolysis, the process comprising: a step of filling metal chloride in an electrolysis vessel having a positive electrode and a negative electrode; a step of heating and fusing the metal chloride to make an electrolytic bath; and a step of electrolyzing the electrolytic bath to deposit metal on the negative electrode in a solid state.
 12. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the temperature of the electrolytic bath used in the molten-salt electrolysis is maintained at not more than the melting point of the metal.
 13. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the temperature of the electrolytic bath used in the molten-salt electrolysis is maintained at not less than the melting point of the metal, and at the same time the temperature of the negative electrode is maintained at not more than the melting point of the metal.
 14. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the metal in a solid state deposited on the negative electrode is scraped by a mechanical means.
 15. The process for production of a metal by molten-salt electrolysis according to claim 12, wherein the metal in a solid state deposited on the negative electrode is scraped by a mechanical means.
 16. The process for production of a metal by molten-salt electrolysis according to claim 13, wherein the metal in a solid state deposited on the negative electrode is scraped by a mechanical means.
 17. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the negative electrode on which the metal in a solid state is deposited is immersed in metal chloride filled in a recovery vessel maintained at a temperature not less than the melting point of the metal, so as to recover the metal.
 18. The process for production of a metal by molten-salt electrolysis according to claim 12, wherein the negative electrode on which the metal in a solid state is deposited is immersed in metal chloride filled in a recovery vessel maintained at a temperature not less than the melting point of the metal, so as to recover the metal.
 19. The process for production of a metal by molten-salt electrolysis according to claim 13, wherein the negative electrode on which the metal in a solid state is deposited is immersed in metal chloride filled in a recovery vessel maintained at a temperature not less than the melting point of the metal, so as to recover the metal.
 20. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the electrolytic bath is divided into an electrolysis chamber and a dissolution chamber by a dividing wall, the positive electrode is arranged in the electrolysis chamber, and the negative electrode is arranged to enable orbital movement in circle through the electrolysis chamber and dissolution chamber, wherein the temperature of the negative electrode is maintained at a temperature not more than the melting point of the metal when the negative electrode passes through the electrolysis chamber and the temperature of the negative electrode is maintained at a temperature not less than the melting point of the metal when the negative electrode passes through the dissolution chamber.
 21. The process for production of a metal by molten-salt electrolysis according to claim 20, wherein the negative electrode comprises plural electrodes and the electrodes are arranged on the orbital path.
 22. The process for production of a metal by molten-salt electrolysis according to claim 11, wherein the metal chloride is calcium chloride and the metal is calcium metal.
 23. An apparatus for production of a metal by molten-salt electrolysis, in which metal chloride is filled in an electrolysis vessel having a positive electrode and a negative electrode, the metal chloride is heated and molten to make an electrolytic bath, and the electrolytic bath is electrolyzed to deposit metal on the negative electrode in a solid state, wherein the electrolytic bath is divided into an electrolysis chamber and a dissolution chamber by a dividing wall, the positive electrode is arranged in the electrolysis chamber, the negative electrode is arranged to enable orbital movement in a circle through the electrolysis chamber and dissolution chamber, and wherein the metal deposited on the negative electrode while the electrode is in the electrolysis chamber is separated and recovered while the electrode is in the dissolution chamber.
 24. The apparatus for production of metal by molten-salt electrolysis according to claim 23, wherein the negative electrode comprises plural electrodes and the electrodes are arranged on the orbital path. 